The gasification of liquids, i.e. the treatment of liquids with gases to dissolve soluble gases in the liquid, is carried out for various purposes, especially for the oxygenation of oxygen-poor liquids, for example, waste water or sewage, oxygen-poor natural water and the like. The oxygenation promotes bacterial action causing decomposition of organic materials and generally improving the liquid. In the treatment of sewages using the oxygenation method, oxygenation promotes the formation of sludge which results in bacterial decomposition of organic matter to leave an effluent which is substantially pure and which can be decanted from this sludge. The latter can be disposed of in various ways.
Natural waters likewise may be oxygen-poor and may be incapable of sustaining aquatic life. Accordingly it has been found to be advantageous to oxygenate this water before it is released into a natural water course.
In the gasification of liquids, i.e. the dissolution of gases in liquids, it is known to improve the residence time or contact time of the gas with the liquid by forming gas bubbles in a column of the liquid which is forced downwardly and thereby circulated within a vessel containing the liquid to be treated. Since the gas bubbles tend to rise in the column, because of their low specific gravity, the downward flow of the column tending to entrain the bubbles downwardly, can promote a long duration of contact between the gas and the liquid. The rate at which the gas rises in the column can be controlled by variation in the velocity of the downward flow of liquid within the column. This technique has been used principally in processes for the oxygen-enrichment of prepurified waste waters.
Especially in large vessels for the oxygenation of waste waters, e.g. in large-scale sewage treatment plants and waste-water treatment facilities, the oxygen consumption and the energy consumption necessary to introduce a particular concentration of soluble oxygen in the liquid are both limited. In other words, there is a maximum solubilization rate for the oxygen and this together with the waste water throughput must be considered when economic operation of the plant is desired.
In a conventional plant, for example, an increase in the throughput of the liquid may reduce the duration of contact of the oxygen with the liquid phase and hence the rate of solubilization. On the other hand, an increase in the gas supply to the liquid within a circulating column within a vessel may not result in an increase in soluble oxygen within the liquid. Furthermore, increased flow rates of the liquid result in increased consumption of energy for accomplishing the higher liquid velocity and hence excessive energy consumption for a given oxygenation of the liquid phase.
Investigations have demonstrated that increased liquid flow velocities in a gasification column sharply decrease the quantities of soluble gases per unit power consumption (kg of gas dissolved per kw hour of energy consumed).